Polyester films, such as films of polyethylene terephthalate, have many advantageous properties, such as excellent strength and dimensional stability, which render them advantageous for use as photographic film supports. In certain applications as photographic film supports, it is necessary that they be provided with an anti-halation layer. The anti-halation layer, which is coated on the side of the support opposite to the image-forming layer(s), functions to prevent light that passes through the film support from being reflected into the image-forming layer(s) and thereby causing an undesired spreading of the image which is known as halation. The anti-halation layer is comprised of an anti-halation agent, typically a dye or pigment which functions to absorb the light, dispersed in an alkali-soluble polymeric binder that renders the layer removable by an alkaline photographic processing solution. In a typical example of a commercially useful anti-halation layer, the light-absorbing material utilized is carbon black and the binder is a cellulose ester such as cellulose acetate hexahydrophthalate.
The photographic art has been faced for many years with a difficult problem in obtaining adequate adhesion of the anti-halation layer to the polyester film. Although the anti-halation layer is removed in processing, it must, of course, be able to remain firmly adhered to the polyester film during normal handling and use prior to such removal. One technique which has met with limited success is to utilize an aqueous film-forming composition containing an anti-halation agent, an acrylate-acrylic acid polymer and ammonium hydroxide. Such compositions and their application to polyester films to form anti-halation layers are described in Thompson et al, U.S. Pat. No. 2,976,168 issued Mar. 21, 1961. Another useful procedure involves application to the polyester film of an aqueous dispersion containing a polymeric binder, an anti-halation agent, and an adhesion promoter. This process is described in Pierce et al, U.S. Pat. No. 3,201,249 issued Aug. 17, 1965. The adhesion promoters are compounds such as resorcinol which funcion as attack agents for the polyester and thereby promote improved adhesion. Use of such an adhesion promoter suffers, however, from serious disadvantages which significantly restrict its commercial utilization. Thus, for example, the amount of resorcinol utilized must be very carefully regulated. If too little is used, the anti-halation layer will tend to come off the polyester support under typical conditions of handling and use. On the other hand, there are also serious disadvantages to the use of too much resorcinol. In particular, resorcinol tends to sublime from the coated layer during drying of the anti-halation layer, and this causes fouling and contamination of the drying equipment and the product. In addition, too much resorcinol promotes such strong adhesion of the anti-halation layer to the polyester support that adequate removal in conventional removal processes cannot be achieved.
It is toward the objective of providing a novel process for the formation of an anti-halation layer on a polyester photographic film support, that avoids the need for use of adhesion promoters such as resorcinol, that the present invention is directed.